When continuously heating a steel strip in a heating furnace, for example, measurement of the surface temperature of the steel strip heated in the heating furnace is very important in controlling the heating operation of the steel strip in the heating furnace.
The surface temperature of a heated steel strip is generally measured with a radiation thermometer, and accurate determination of emissivity of the surface of the steel strip is essential for the accurate measurement of the surface temperature of the steel strip.
A method for measuring the surface temperature of a heated steel strip through calculation of the surface emissivity of the heated steel strip is disclosed in a paper entitled "A Method for Simultaneous Measurement of Both Temperature and Emissivity, and Its Applications to Steel Processes", released in the technical journal "Tetsu-to-Hagane" (which means "Iron and Steel") and published in Japan on Jan. 1, 1979 (hereinafter referred to as the "prior art"). The prior art is described below with reference to the drawings.
FIG. 1 is a descriptive front view illustrating the method for measuring the surface temperature of a heated steel strip of the prior art, and FIG. 2 is a descriptive plan view of FIG. 1. As shown in FIGS. 1 and 2, a cylindrical reflecting plate 1 is vertically provided above the surface of a heated steel strip 2, spaced apart from the surface thereof. The inner surface of the cylindrical reflecting plate 1 is plated with gold so as to efficiently reflect heat radiation emitted from the surface of the steel strip 2. A radiation thermometer 3 is provided above the cylindrical reflecting plate 1 and on the axial line of the cylindrical reflecting plate 1. A rotary chopper 4 having a pair of slits 5 is horizontally provided adjacent to the top end of the cylindrical reflecting plate 1. The rotary chopper 4 rotates at a constant speed by a motor 6 and opens and closes the upper opening of the cylindrical reflecting plate 1.
As shown in FIG. 3(a), when the rotary chopper 4 opens the upper opening of the cylindrical reflecting plate 1, an amount of energy E.sub.1 of a heat radiation which is emitted from a point P on the surface of the steel strip 2 and comes directly into the radiation thermometer 3, is measured by the radiation thermometer 3. Then, as shown in FIG. 3(b), when the rotary chopper 4 closes the upper opening of the cylindrical reflecting plate 1, the total sum E.sub.2 of the amount of energy E.sub.1 of the heat radiation emitted from the point P and an amount of energy of a heat radiation which is emitted also from the point P and comes into the radiation thermometer 3 through the slit 5 of the rotary chopper 4 after having been reflected several times among the surface of the steel strip 2, the inner surface of the cylindrical reflecting plate 1 and the inner surface of the rotary chopper 4, is measured by the radiation thermometer 3. The amount of energy E.sub.2 is larger than the amount of energy E.sub.1, and these are expressed by the following formulae: EQU E.sub.1 =.epsilon..multidot.E.sub.b (T) (1), and EQU E.sub.2 =g(.epsilon.).multidot.E.sub.b (T) (2)
where,
.epsilon.: emissivity of the surface of the steel strip 2, PA1 E.sub.b (T): amount of energy of heat radiation emitted from the surface of a perfect blackbody having a temperature T (in other words, E.sub.b (T) means the amount of energy of a reference heat radiation), and PA1 g(.epsilon.): apparent emissivity of the surface of the steel strip 2. PA1 .alpha.: constant dependent upon the shape and the reflection characteristics of the cylindrical reflecting plate 1.
Since the amount of energy E.sub.2 is larger than the amount of energy E.sub.1, as described above, the apparent emissivity g(.epsilon.) is larger than the emissivity .epsilon.. The emissivity .epsilon. can be calculated by the following formula on the basis of the mutual reflection theory: EQU g(.epsilon.)=.epsilon.(.alpha.+1)/(.epsilon.+.alpha.) (3)
where,
Therefore, Formula (2) can be expressed as follows: EQU E.sub.2 =E.sub.b (T).multidot..epsilon.(.alpha.+1)/(.epsilon.+.alpha.) (4)
From Formulae (1) and (4), the emissivity .epsilon. can be expressed as follows: ##EQU1##
From Formulae (1) and (5), E.sub.b (T) can be expressed as follows: ##EQU2##
In Formula (6), E.sub.1, E.sub.2 and .alpha. are known. It is therefore possible, by means of Formula (6), to calculate the amount of energy E.sub.b (T) of heat radiation emitted from the surface of the steel strip 2 on the assumption that the steel strip 2 is a perfect blackbody. Since the temperature corresponding to the amount of energy E.sub.b (T) is previously known, the surface temperature of the steel strip 2 can be determined from the amount of energy E.sub.b (T).
The above-mentioned prior art involves however the following problems: Because of the complicated paths of reflection of the heat radiation among the surface of the steel strip 2, the inner surface of the cylindrical reflecting plate 1 and the inner surface of the rotary chopper 4, it is impossible to calculate the number of reflections of the heat radiation, leading to complicated calculations for determining the surface temperature of the steel strip 2. In addition, because of the necessity of providing the rotary chopper 4 adjacent to the top end of the cylindrical reflecting plate 1, the temperature measuring apparatus requires a large space, resulting in restrictions on the site of installation of the apparatus.
Under such circumstances, there has been a demand for the development of a method allowing simpler measurement of the surface temperature of a heated steel strip and requiring a smaller space for the temperature measuring apparatus, but such a method has not as yet been proposed.